1. Field of the Invention
This invention relates to temperature compensating arrangements for correcting frequency variations due to temperature changes in systems employing crystal oscillators.
2. Description of the Prior Art
Hitherto, a crystal oscillator has been widely used as a circuit for deriving a signal of a constant frequency. However, for achieving a signal of such constant frequency, it is required to compensate a temperature characteristic of a crystal. Two methods for compensating the aforesaid temperature characteristic are known, i.e., (i) one to place a crystal in a thermostatic oven while controlling a heater current of the oven so as to maintain the temperature of the oven constant, and (ii) to add a capacitance of a variable capacitor, such as a variable capacitance diode, to a crystal for controlling a capacitance in response to temperature changes so as to maintain a constant oscillating frequency.
According to the second case there is used a differential amplifier which detects an unbalanced voltage produced due to a temperature change by use of a bridge circuit including a temperature sensitive element such as a thermistor. The capacitance of a variable capacitor is then controlled according to an output voltage thus detected for temperature compensation of an oscillation frequency. However, the aforesaid variable capacitor has a temperature characteristic which affords an extremely narrow range of approximate linearity, so that temperature compensation only remains possible in an extremely narrow range.